Undelivered Mail Returned to Sender while customer is inactive

[MusikMachineMan]

Hello,

i use a birthday module which send a voucher to customers on their date of birth

and i daily receive lookalike emails Undelivered Mail Returned to Sender :

"This is the mail system at host mo804.mail-out.***.net. I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. For further assistance, please send mail to postmaster."

the thing is it's an important thing to know that the email adress of a customer is not anymore active, but the problem is that sometimes, the customer is already inactive in my database, with newsletter and opt-in selected OFF !!

how could it still send emails to inactive customers ?

many thanks for your help